Power amplifiers are susceptible to damage when subjected to extreme conditions such as high temperatures. High temperatures can have various causes, including extreme current due to mismatched loads at the antenna, high duty cycle from data transmission, battery regulation when recharging a battery, or high ambient temperatures.
A typical RF power amplifier can be specified to handle load mismatches on the order of a 10:1 ratio. A mismatch of 10:1 implies that a power amplifier must drive loads of 5 ohms to 500 ohms. Nominally, the RF amplifier is specified is specified to drive a 50 ohm load. At the extremes of these mismatched conditions, a power amplifier, regardless of which process technology has been implemented, could experience stresses that have the potential to cause permanent damage to the power amplifier.
In a power amplifier implemented using a complementary metal oxide semiconductor (CMOS), when the load impedance at the antenna is high (e.g., 500 ohms), the node voltages could exceed the CMOS breakdown voltage and cause permanent damage. When the load impedance of a load is low (e.g., 5 ohms), the current supplied to the load may be high enough to result in thermal problems. This invention addresses the problems associated with thermal issues.
Some prior art power amplifiers use current sensors to prevent the power amplifiers from supplying too much current to a load. In one example, current is sensed external to the power amplifier, and the power amplifier is turned off when a current limit is exceeded. One problem with this approach is that using only current sensing may inhibit certain modes of operation which are acceptable, such as a high current for a short period of time. Also, some current sensing techniques do not take into account environmental conditions such as ambient temperature, ventilation, heat sinking, etc. Another problem with this approach relates to the fact that the system only knows the current drawn, which can result in flaws. For example, such a system can not take into account the efficiency of the power amplifier or the true junction temperature of the die. Such a system is also inefficient, since the resistor will dissipate power, thus reducing overall efficiency.